Traditional computer systems (e.g., desktop personal computers and server computers) used a single Industry Standard Architecture (ISA) interconnect to connect devices (e.g., keyboard and mouse). The Peripheral Components Interconnect (PCI) interconnect was developed to connect other devices (e.g., video) via an adapter card to the processor. A Host/PCI interconnect bridge typically connects the host interconnect and the PCI interconnect. All of these interconnects were intended for a direct connection between the device and the CPU and were not intended to be shared.
Today disaggregated server architecture is a thing of the super computer. However, virtualization is driving toward more resource sharing and pooling into the mainstream/volume servers. It is expected that mainstream providers will be introducing server disaggregation architectures in the coming years. Efforts around standardization can be seen from within industry standard bodies such as the PCI-SIG's IOV workgroup.
The PCI interconnect was conceived as a Local IO interconnect, and as such it was created to link devices within a single host (in the box architecture). The cost and speeds of PCI make it an attractive solution for disaggregating the system architecture. However, PCI has a number of challenges to overcome including its rooted architecture, ordering rules and legacy programming model as a result a number of unsuccessful attempts to extended PCI. For example, disaggregated volume systems do not include interconnect connectors, such as a PCI connector, because of a perceived difficulty in extending the PCI interconnect across the backplane.